The present invention relates to a GPS signal transmitter and a signal transmission method thereof and, particularly, to a GPS signal transmitter located in a space where radio waves from satellites are not receivable and a signal transmission method thereof.
A GPS (Global Positioning System) is often used to acquire location information. The GPS is one of positioning systems utilizing PGS satellites. In the GPS, a receiver receives positioning signals including time information from GPS satellites orbiting the earth at an altitude of about 20,000 kilometers and is able to know its location (in terms of latitude, longitude, and altitude) on the earth by calculation from the received positioning signals. The GPS is the system developed in USA.
Positioning systems utilizing GPS satellites are generally called GNSSs (Global Navigation Satellite Systems). Among GNSSs, besides the GPS that is currently operating, some positioning systems are expected to operate in future, such as GLONASS in Russian Federation, Galileo in European Union, and a Quasi-zenith Satellite System in Japan. In this specification, satellite positioning systems are generically referred to as GPS.
For positioning by GPS, it is necessary to receive time information transmitted from a GPS satellite. Therefore, if a receiver is located in environments such as in a tunnel, underground, and in-premises spaces where a receiver cannot receive a signal including time information with a required intensity (hereinafter, a term “in-premises” is used to refer to such environments), the receiver is not able to acquire location information with a required accuracy.
Japanese Patent Application Laid-Open Publication No. 2007-278756 discloses a method for providing location information in in-premises space where positioning signals from GPS satellites are not receivable. In Japanese Patent Application Laid-Open Publication No. 2007-278756, specifically, a device located in premises to transmit a signal compliant with a GPS navigation message (this device will be a GPS signal transmitter hereinafter) transmits location information to a device capable of receiving a GPS signal (the latter device will be referred to as a GPS signal receiver hereinafter). The GPS signal receiver identifies its location by acquiring the GPS signal transmitter's location information.
Another location technique in premises is disclosed in Japanese Patent Application Laid-Open Publication No. H10-48317. In this technique, fixed receiving stations transmit data received from GPS satellites and their location information in space inside premises. A user mobile terminal receives the thus transmitted data from the GPS satellites, calculates distances from the GPS satellites via the fixed receiving stations, and calculates distances between the GPS satellites and the fixed receiving stations, based on the fixed stations' location information received and the positions of the GPS satellites. From these results, the user mobile terminal calculates the distances from each fixed receiving station and determines its location.
“User Interface Specifications of Quasi-zenith Satellite System” (available on the Internet at <URL:http://qzss.jaxa.jp/is-qzss/IS-QZSS—10_J.pdf>) specifies the transmission scheme, format, signal output, etc. of a terrestrial complementary signals (IMES (Indoor Messaging System) signal); it is assumed in Japanese Patent Application Laid-Open Publication No. 2007-278756 that the GPS signal transmitter transmits this signal.